Messages in this thread | | | Date | Wed, 03 Apr 2013 10:50:57 -0700 | From | John Stultz <> | Subject | Re: [RFC] perf: need to expose sched_clock to correlate user samples with kernel samples |
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On 04/03/2013 10:35 AM, Pawel Moll wrote: > On Wed, 2013-04-03 at 18:29 +0100, John Stultz wrote: >> On 04/03/2013 10:19 AM, Pawel Moll wrote: >>> On Tue, 2013-04-02 at 17:19 +0100, John Stultz wrote: >>>> But if we're going to have to do >>>> this via a clockid, I'm going to want it to be done via a dynamic posix >>>> clockid, so its clear its tightly tied with perf and not considered a >>>> generic interface (and I can clearly point folks having problems to the >>>> perf maintainers ;). >>> Ok, so how about the code below? >>> >>> There are two distinct parts of the "solution": >>> >>> 1. The dynamic posix clock, as you suggested. Then one can get the perf >>> timestamp by doing: >>> >>> clock_fd = open("/dev/perf-clock", O_RDONLY); >>> clock_gettime(FD_TO_CLOCKID(clock_fd), &ts) >>> >>> 2. A sort-of-hack in the get_posix_clock() function making it possible >>> to do the same using the perf event file descriptor, eg.: >>> >>> fd = sys_perf_event_open(&attr, -1, 0, -1, 0); >>> clock_gettime(FD_TO_CLOCKID(fd), &ts) >> #2 makes my nose wrinkle. > To make myself clear: I consider the code as it is a hack. > >> Forgive me for being somewhat ignorant on the >> perf interfaces, but why is the second portion necessary or beneficial? > My thinking: the perf syscall returns a file descriptor already, so it > would make sense to re-use it in the clock_gettime() call instead of > jumping through loops to open a character device file, which may not > exist at all (eg. no udev) or may be placed or named in a random way > (eg. some local udev rule). > > I'm open for different opinions :-)
Cc'ing Richard for his thoughts here.
I get the reasoning around reusing the fd we already have, but is the possibility of a dynamic chardev pathname really a big concern?
I'm guessing the private_data on the perf file is already used?
Maybe can we extend the dynamic posix clock code to work on more then just the chardev? Although I worry about multiplexing too much functionality on the file.
thanks -john
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